A pressed-wood composite product can be produced from a prepared pre-assembly mat which includes selected wood components along with intercomponent, heat-curable adhesive. A typical end product may, for example be plywood, or laminated veneer lumber (LVL), which, after production can be cut for use, or otherwise employed, in various ways as wood-based building components. The starter material would typically be, in addition to a suitable heat-curable adhesive, (a) thin sheet veneers of wood, (b) oriented strands (or other fibrous material) of smaller wood components, (c) already pre-made expanses of plywood which themselves are made up of veneer sheets or (d) other wood elements.
In conventional LVL fabrication processing, LVL is typically made of glued, veneer sheets of natural wood, utilizing adhesives, such as urea-formaldehyde, phenol, resolsenidi, formaldehyde formulations which require heat to complete a curing process or reaction. There are several well-known and widely practiced methods of manufacturing and processing to create LVL. The most common pressing technology involves a platen press, and a method utilizing such a press is described in U.S. Pat. No. 4,638,843. Pressing and heating is typically accomplished by placing precursor LVL between suitable heavy metal platens. These platens, and their facially “jacketed” wood-component charges, are then placed under pressure, and are heated with hot oil or steam to implement the fabrication process. Heat from the platens is slowly transferred through the wood composite product, the adhesive cures after an appropriate span of pressure/heating time. This process is relatively slow, the processing time increasing with the thickness of the product.
U.S. Pat. No. 5,628,860 describes an example of a technique wherein radio frequency (RF) energy is added to the environment within (i.e., in between) opposing press platens to accelerate the heating and curing process and thereby shorten fabrication times.
Still another technique to provide the heating and curing is to utilize microwave energy. U.S. Pat. No. 5,895,546, discloses use of microwave energy to preheat loose LVL lay-up materials, which are then finished in a process employing a hot-oil-heated, continuous-belt press. Also CA 2 443 799 discloses a microwave preheat press. A microwave generator feeds through a waveguide a microwave applicator such the microwave energy is applied to an initial press section which leads into a final press section. Multiple waveguides in a staggered configuration may be used to provide multiple points of application of the microwave energy with a waveguide spacing that yields substantially uniform heating pattern. Heating temperature is adjusted by varying the linear feed rate at which the wood element enters the microwave preheat press, or by controlling the microwave waveform.
EP0940060 discloses another microwave preheat press wherein the microwave energy is feed through waveguide to applicators on both sides of the wood product. The feeding waveguides are provided with sensor for measuring reflected microwave energy, and a tuner section for generating an induced reflection which cancels the reflected energy. The tuner section includes tuning probes whose length within the feeding waveguides are adjusted by a stepper motor.
U.S. Pat. No. 6,744,025 discloses a microwave heating unit formed into a box-like resonant cavity via which the product to be heated is passed. The product is passed via a narrow gap that extends lengthwise through the entire cavity and divides the cavity substantially at the midline of the cavity into two opposed subcavities. The microwave energy to be imposed on the product is fed via a waveguide to one of the subcavities.
U.S. Pat. No. 7,145,117 discloses an apparatus for heating a board product containing glued wood. The apparatus comprises a heating chamber through which the board product passes and in which a microwave heating electrical field is provided to prevail substantially on the board plane, in transversal direction with respect to the proceeding direction of the board, by means of a microwave frequency energy applied perpendicular to the board plane.
GB893936 discloses a microwave heating apparatus wherein a resonant cavity is formed by a segment of a standard waveguide which is a rectangular in transverse cross-section with a longer side and a shorter side. The cavity is coupled to the waveguide through an adjustable matching iris forming one end of the cavity. The cavity can be tuned by means of an adjustable short circuiting piston serving as the other end wall of the cavity. Two opposite longer sides of the standard waveguide cavity are further provided with slots extending lengthwise of the cavity to allow a planar product pass through the cavity between adjustable side plates located on the opposite shorter sides of the cavity. The side plates shorten the longer sides of the cavity with respect to the respective sides of the standard waveguide such that the waveguide segment of cut-off frequency close to an operating frequency is formed. End parts of the cavity beyond the side plates have cross-sectional dimensions of the standard waveguide. A sensor is provided to measure the energy reflected from the cavity. The frequency is tuned so that the energy reflected from the cavity is a minimum. Side plates are then adjusted so as to produce a uniform field across the width of the planar product to be heated. This prior art structure has various drawbacks.
1. The prior art structure is suitable only for heating products with very limited cross-section. The thickness of the heated product shall not exceed 10 to 15% of length of the longer side of the standard waveguide. The width of the heated product (along the longitudinal axis of the cavity) should not be longer than length of the longer side of the standard waveguide.
2. The heating occurs on a distance (along the direction of movement of the heated product) that is equal to the length of the shorter side of the waveguide.
3. Losses in the waveguide metal increases strongly when the operating frequency goes to the cut-off frequency of the waveguide.
4. The cavity has a low Q factor. Insertion of the material to be heated into the cavity will additionally degrade the Q factor of the cavity. This results in non-uniform heating pattern and destruction of the resonant phenomenon.
Also GB1016435 discloses a microwave heating apparatus intended to improve the structure of GB893936. GB1016435 notes as a disadvantage of GB893936 that adjustment of the tuning plunger and adjustment of the iris affect not only the tuning of cavity but also the standing wave pattern in the cavity, and this complicates the provision of the desired uniform distribution of the electric field along the central part of the cavity. In GB1016435, a resonant cavity is formed by a waveguide having a rectangular cross-section with a longer side and a shorter side. The microwave energy is supplied into the cavity by means of a coaxial feeder and a coupling loop. The tuning of the cavity is performed by metal rods which extend lengthwise of the cavity. The waveguide or cavity terminates at each end in an effective open-circuit formed by a waveguide section having larger cross-sectional dimensions than the central cavity section. With this structure, the field intensity along the central cavity is alleged to be substantially uniform along the heating area. However, the structure of GB1016435 has the same disadvantages as listed for GB893936 above. Moreover, tuning by means of a metal rod is questionable, because the metal rod may create with the walls of the waveguide cavity a TEM transmission line of substantially different wavelength than the waveguide, and it may further degrade heating uniformity.